U.S. patent number 6,219,498 [Application Number 09/494,428] was granted by the patent office on 2001-04-17 for image forming apparatus having improved image transfer characteristics.
This patent grant is currently assigned to Sharp Kabushiki Kaisha. Invention is credited to Atsuyuki Katoh, Yoshiya Kinoshita, Kenji Sugimura, Hiroshi Tachiki, Masaru Tsuji.
United States Patent |
6,219,498 |
Katoh , et al. |
April 17, 2001 |
Image forming apparatus having improved image transfer
characteristics
Abstract
By application of a voltage from a power source, a transfer
roller carries out both transfers of (A) a first transfer for
transferring a toner image on a photoreceptor to an intermediate
transfer medium and (B) a second transfer for transferring
overlapped color toner image on the intermediate transfer medium at
once onto a recording sheet. A transfer voltage in accordance with,
for example, a sheet thickness, as detected by sheet type detection
of a sheet type detecting section is applied to the transfer roller
in each stage. When the sheet type detecting section detects that
the recording sheet is longer than a periphery of a fixing belt, a
heater lamp is turned on in advance at any instant between a time
when the recording sheet is brought into contact with the fixing
roller and a time when the fixing roller completes one rotation,
taking into consideration a time required for conduction of heat
from the heater lamp to the fixing belt. As a result, it is
possible to reduce ozone generated and make the device compact, and
desirable transfer characteristics in accordance with a sheet type
are obtained, thus realizing uniform fixing of an unfixed toner
image onto the recording sheet, from the front end to the rear
end.
Inventors: |
Katoh; Atsuyuki (Tenri,
JP), Tsuji; Masaru (Nara, JP), Sugimura;
Kenji (Yamatokoriyama, JP), Kinoshita; Yoshiya
(Ikoma-gun, JP), Tachiki; Hiroshi (Yamatokoriyama,
JP) |
Assignee: |
Sharp Kabushiki Kaisha (Osaka,
JP)
|
Family
ID: |
18216990 |
Appl.
No.: |
09/494,428 |
Filed: |
January 31, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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195543 |
Nov 19, 1998 |
6044236 |
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Foreign Application Priority Data
|
|
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|
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Nov 28, 1997 [JP] |
|
|
9-329044 |
|
Current U.S.
Class: |
399/45; 399/389;
399/69 |
Current CPC
Class: |
G03G
15/0131 (20130101); G03G 15/1675 (20130101); G03G
15/5029 (20130101); G03G 15/161 (20130101); G03G
21/0005 (20130101); G03G 15/2039 (20130101); G03G
2215/018 (20130101); G03G 2215/0196 (20130101); G03G
2215/1614 (20130101); G03G 2215/1661 (20130101); G03G
2215/2016 (20130101); G03G 2215/2032 (20130101) |
Current International
Class: |
G03G
15/16 (20060101); G03G 15/20 (20060101); G03G
15/01 (20060101); G03G 015/00 (); G03G
015/20 () |
Field of
Search: |
;399/45,68,69,66,50,51,302,389 |
References Cited
[Referenced By]
U.S. Patent Documents
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5204729 |
April 1993 |
Maeda et al. |
5438398 |
August 1995 |
Tanigawa et al. |
5666599 |
September 1997 |
Miyasaka et al. |
5729808 |
March 1998 |
Bisaiji et al. |
5745255 |
April 1998 |
Yamaguchi |
5745830 |
April 1998 |
Fujiwara et al. |
5761568 |
June 1998 |
Haragakiuchi et al. |
5809373 |
September 1998 |
Yoda et al. |
5815775 |
September 1998 |
Miyasaka et al. |
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3-55585 |
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Mar 1991 |
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JP |
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4-066970 |
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JP |
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5-265300 |
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Oct 1993 |
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JP |
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6-83144 |
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Mar 1994 |
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JP |
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7-72694 |
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Mar 1995 |
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JP |
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8-22203 |
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Jan 1996 |
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JP |
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8-063052 |
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Mar 1996 |
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JP |
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8-220902 |
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Aug 1996 |
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JP |
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9016001 |
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Jan 1997 |
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JP |
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9-96942 |
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Apr 1997 |
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JP |
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9127805 |
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May 1997 |
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JP |
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9146389 |
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Jun 1997 |
|
JP |
|
9-222823 |
|
Aug 1997 |
|
JP |
|
Primary Examiner: Pendegrass; Joan
Parent Case Text
This application is a divisional of application Ser. No.
09/195,543, filed on Nov. 19, 1998, now U.S. Pat. No. 6,044,236,
the entire contents of which are hereby incorporated by reference.
Claims
What is claimed is:
1. An image forming apparatus, comprising:
latent image holding means for holding color-separated image
information as an electrostatic latent image;
a plurality of developing means for making the electrostatic latent
image held by said latent image holding means visible color by
color;
an intermediate transfer medium to which a visualized image of each
color visualized on a surface of said latent image holding means is
successively transferred upon contact with said latent image
holding means;
transfer means for transferring the visualized image successively
transferred onto said intermediate transfer medium to the recording
medium; and
voltage applying means for applying a predetermined voltage to said
transfer means,
said image forming apparatus further comprising:
sheet type detecting means for detecting a type of the recording
medium; and
control means for controlling a parameter associated with an image
forming operation in accordance with a detection signal from said
sheet type detecting means,
said sheet type detecting means including transmission detecting
means for detecting light transmitted through the recording medium
and reflection detecting means for detecting light reflected off a
surface of the recording medium;
a wavelength of light projected on the recording medium for
detection by the transmission detecting means being set such that a
characteristic of transmittance against wavelength is sorted out in
accordance with a mass per unit area of the recording medium.
2. The image forming apparatus as set forth in claim 1, wherein the
parameter is the predetermined voltage applied to said transfer
means.
3. The image forming apparatus as set forth in claim 1, wherein the
parameter is a transport speed of the recording medium.
4. The image forming apparatus as set forth in claim 1, wherein the
parameter is a fixing temperature of fixing an unfixed visualized
image transferred on the recording medium.
5. The image forming apparatus as set forth in claim 1, wherein the
parameter is a development bias voltage.
6. The image forming apparatus as set forth in claim 1, wherein the
parameter is an image forming rate.
7. The image forming apparatus as set forth in claim 1, wherein the
parameter is a voltage for charging said latent image holding
means.
8. The image forming apparatus as set forth in claim 1, wherein the
parameter is a voltage of a power source for applying a voltage to
a sheet removal charger which removes the recording medium from
said intermediate transfer medium.
9. The image forming apparatus as set forth in claim 1, wherein the
type of the recording medium detected by said sheet type detecting
means is at least one kind selected from the group consisting of
transparency, thickness, length, and color.
10. The image forming apparatus as set forth in claim 1, further
comprising:
fixing means, which is rotatable, for fixing an unfixed visualized
image transferred on the recording medium;
heating means for heating said fixing means so that a surface
temperature of said fixing means becomes a predetermined
temperature; and
fixing temperature control means for controlling an on-and-off
state of said heating means in accordance with a type of the
recording medium,
wherein said fixing temperature control means carries out a
control, when a length of the recording medium detected by said
sheet type detecting means is longer than a periphery of said
fixing means, so that said heating means is turned on in advance at
any instant between a time when the recording medium is brought
into contact with said fixing means and a time when said fixing
means completes one rotation, taking into consideration a time
required for conduction of heat from said heating means to said
fixing means.
Description
FIELD OF THE INVENTION
The present invention relates to an image forming apparatus such as
a copying machine and a laser printer, and particularly to an image
forming apparatus which transfers a color toner image formed on a
photoreceptor at once onto a recording medium such as paper via an
intermediate transfer medium.
BACKGROUND OF THE INVENTION
Conventionally, a variety of image forming apparatuses such as a
copying machine have been proposed. Such image forming apparatuses
are roughly divided into two types by a difference in transfer
system of a color toner image onto a recording medium. One is an
image forming apparatus adopting a so-called direct transfer system
in which a recording medium is held by being wrapped around a
transfer medium, and a toner image on a photoreceptor is directly
transferred onto the recording medium color by color. The other is
an image forming apparatus in which a color toner image formed on a
photoreceptor is transferred at once onto an intermediate transfer
medium, and thereafter transferred onto a recording medium from the
intermediate transfer medium.
However, in the former image forming apparatus, there is a case
where a desired color image cannot be obtained depending on the
type of a recording medium used. This is because of the fact that
the transfer characteristics of the toner image are different for
different colors depending on the type (especially thickness) of
the recording medium used. As a result, in a color toner image
composed of overlapping toner images of different colors, the
coloring differs among recording media. Thus, in order to obtain a
desired color image in this image forming apparatus, it is required
to limit the range of thickness, etc., of the recording medium
used.
However, in recent years, a demand for color copying for a
recording medium of various thicknesses and various paper types has
come to a level which cannot be met by the described image forming
apparatus. Further, in the above image forming apparatus, because
the toner image is transferred to the recording medium color by
color, the overlapping accuracy of the toner images is always a
problem. However, it has come to a level where the overlapping
accuracy cannot be improved any further.
Therefore, it now has been a focus of ongoing research to develop
an image forming apparatus provided with an intermediate transfer
medium, which is capable of color copying regardless of the type of
a recording medium used. As an example of such an image forming
apparatus, the image forming apparatus as disclosed in Japanese
Unexamined Patent Publication No. 251864/1991 (Tokukaihei 3-251864)
is described below. In this image forming apparatus, a copying
process is carried out for each of a high density portion and a low
density portion of a single document image. As a result, a color
toner image obtained by a single copying process is overlapped with
another color toner image on the intermediate transfer medium, thus
forming a single color toner image.
The above image forming apparatus as a copying machine as disclosed
in the above publication is provided with, as shown in FIG. 27, a
transparent document plate 101 on an upper surface. Below the
document plate 101 is provided an exposure optical system 103 for
exposing and scanning a document 102 and exposing a photoreceptor
104 (mentioned later).
The exposure optical system 103 is provided with a light source
lamp 103a for projecting light onto the document 102 placed on the
document plate 101, a plurality of reflecting mirrors 103b through
103f for guiding, as shown by the alternate short and long line in
FIG. 27, the reflected light off the document 102 onto the
photoreceptor 104, a focus lens 103g provided in the path of the
reflected light, and a color separating filter composed of color
filters of three primary colors of red, green, and blue.
Underneath the exposure optical system 103 is provided the
photoreceptor 104 having a belt shape. The photoreceptor 104 is
suspended between two rollers 105 and 106, which are placed with a
certain gap therebetween, and the photoreceptor 104 is rotatably
driven by a motor (not shown).
Around the photoreceptor 104 on the roller 106 side are provided,
along with other members, a static charger 107 for charging the
photoreceptor 104, a cleaning device 108 for removing toner
remaining on the photoreceptor 104, and a screen filter 109 for
splitting the reflected light off the document 102 into rays.
On the upper side of the photoreceptor 104 is provided a developing
device 113 having three developer tanks 110 through 112 without
contacting the photoreceptor 104. The developer tanks 110 through
112 store color developers of yellow, magenta, and cyan,
respectively, which are complementary colors of the three primary
colors of the color filters of the color separating filter 103h.
The developer tanks 110 through 112 are provided with magnet
rollers 110a through 112a, respectively, which give the respective
color developers to the photoreceptor 104.
Below the photoreceptor 104 are provided sheet feeding cassettes
114 and 115 on top of the other having different sizes for feeding
a recording sheet 130 as a recording medium. On the discharge sides
of the sheet feeding cassettes 114 and 115, sheet feeding rollers
116 and 117 are provided, respectively. In front of the sheet
feeding cassettes 114 and 115 are provided timing rollers 118 for
temporarily stopping the recording sheet 130 so that the recording
sheet 130 is supplied at a predetermined timing.
On the roller 105 side of the photoreceptor 104 is provided an
intermediate transfer device 119. The intermediate transfer device
119 is composed of, along with other members, the intermediate
transfer medium 120 having a belt shape, three rollers 121 through
123 for rotatably driving the intermediate transfer medium 120, a
transfer charger 124 for transferring a toner image of each color
component on the photoreceptor 104 onto the intermediate transfer
medium 120, a transfer charger 125 for transferring a color toner
image formed on the intermediate transfer medium 120 onto the
recording sheet 130, a separating charger 126 for separating the
recording sheet 130 from the intermediate transfer medium 120, and
the cleaning device 127 for removing toner remaining on the
intermediate transfer medium 120.
In the discharge direction of the intermediate transfer medium 120
are provided a transport belt 128 for transporting the recording
sheet 130 and a fixing device 129 for fixing the color toner image
onto the recording sheet 130.
When carrying out full-color copying in the described arrangement,
first, the screen filter 109 is set aside from the exposure path
and the exposure is started with respect to a high density
portion.
Specifically, the light source lamp 103a projects light onto the
document 102 placed on the document plate 101 so as to carry out
optical-scan three times. The reflected light off the document 102
is incident on the color separating filter 103h via the reflecting
mirrors 103b through 103d and the focus lens 103g, and is separated
into color components by the color separating filter 103h. The
reflected light separated into color components is then
successively projected, via the reflecting mirrors 103e and 103f,
onto the photoreceptor 104, which has been uniformly charged by the
static charger 107, so as to expose the photoreceptor 104. As a
result, an electrostatic latent image of each color component,
corresponding to the document image is formed on the photoreceptor
104.
The electrostatic latent image of each color is made visible by
being developed by the corresponding developers of yellow, magenta,
and cyan of the developing device 113, which are complementary
colors of the three primary colors of the color filters of the
color separating filter 103h, and the electrostatic latent image
becomes a toner image. Then, in the intermediate transfer device
119, the toner image of each color component is successively
transferred onto the intermediate transfer medium 120 by the
transfer charger 124 so as to be overlapped. This completes a
single color toner image with respect to the high density portion,
and a first copying process with respect to the high density
portion is finished.
Then, for exposure of a low density portion, the screen filter 109
is introduced into the light path of the light from the exposure
optical system 103, and the optical scan is carried out in the
described manner. Namely, in the exposure with respect to the low
density portion, the reflected light off the document 102 is
projected onto the photoreceptor 104 after being split into rays by
the screen filter 109, thus exposing the photoreceptor 104.
Then, the electrostatic latent image formed by exposure is
developed into a toner image of each color component. The toner
image formed in this manner is successively transferred onto the
color toner image formed on the intermediate transfer medium 120 in
the previous transfer process, thus forming another color toner
image. In this manner, a complete color toner image is obtained
from two color toner images, as obtained from the low density
portion and the high density portion, overlapping with each
another.
The color toner image formed on the intermediate transfer medium
120 is then transferred by the transfer charger 125 onto the
recording sheet 130 which has been supplied from either one of the
sheet feeding cassettes 114 and 115. The recording sheet 130 is
then separated from the intermediate transfer medium 120 by the
separating charger 126 and is guided to the fixing device 129 by
the transport belt 128, and the color toner image is heat-fixed in
the fixing section 129.
The fixing device 129 is usually provided with a heat roller for
heat-fixing the toner image on the recording sheet 130. The surface
temperature of the heat roller is controlled to be a set
temperature by the ON/OFF operation of a heater lamp. FIG. 28 shows
a normal fixing temperature curve when the heater lamp is turned
on.
As shown in FIG. 28, when the heater lamp is turned on, the surface
temperature of the heat roller gradually increases to the set
temperature. When the set temperature is reached, the heater lamp
is turned off, but the temperature continues to rise by the
remaining heat. When the surface temperature drops below the set
temperature, the heater lamp is turned on again. This process is
repeated subsequently, and this results in overshoot in which the
surface temperature fluctuates.
FIG. 29 shows a change in fixing ability with time. As shown in
FIG. 29, the fixing ability is stable on the recording sheet 130
from the front end to the point in length corresponding to the
periphery of the heat roller. However, the fixing ability abruptly
decreases from the point past the periphery of the heat roller to
the rear end of the recording sheet 130. The is because in one
rotation of the heat roller, the heat of the heat roller is given
off to the recording sheet 130 or the toner to be fixed.
In order to prevent this decrease in fixing ability, as shown in
FIG. 30, it has been conventional practice to carry out a control
so that the heater lamp is turned on just when the surface
temperature of the heat roller starts to fall below the set
temperature.
Incidentally, in the described copying machine, transfer of the
toner image of each color component from the photoreceptor 104 to
the intermediate transfer medium 120 is carried out by the corona
discharge of the transfer charger 124. Likewise, transfer of color
toner image from the intermediate transfer medium 120 to the
recording sheet 130 is also carried out by the corona discharge of
the transfer charger 125.
In this kind of corona discharge, the oxygen molecules in the
atmosphere are ionized and ozone is generated. Generally, ozone is
toxic, and in high concentration, damages the respiratory system,
and even a trace amount, when inhaled for an extended period of
time, is fatal. Thus, considering environmental friendliness,
generation of ozone is not preferable.
From this point of view, the described copying machine, provided
with two corona dischargers, which are a source of ozone, lacks
consideration for environmental friendliness.
Also, in the described copying machine, the transfer voltage
applied by the transfer charger 125 is constant regardless of the
type of the recording sheet 130 used. Therefore, there is a case
where desirable transfer is obtained in one recording sheet 130
while transfer failure results when another recording sheet 130
having a different thickness is used. Especially, when thin
recording sheet 130 is used, there is a case where re-transfer
(back-transfer) results, in which the color toner image transferred
on the recording sheet 130 is transferred again onto the
intermediate transfer medium 120 when removing the recording sheet
130 from the intermediate transfer medium 120. Thus, in the above
copying machine, because the transfer voltage is constant, a
desirable transfer characteristic in accordance with sheet type is
not obtained, and as a result, the printing quality suffers.
Conventionally, a copying machine having a function of changing the
transfer voltage in accordance with the sheet feeding cassette
storing recording sheets has been available. However, even when the
recording sheets stored in the same sheet feeding cassette have the
same size, the basis weight (corresponding to thickness) may not be
the same. Further, the recording sheets having the same size may be
transparent or non-transparent. Thus, a transfer characteristic in
accordance with sheet type is not realized even with this
arrangement.
Also, in the conventional fixing mechanism, a control is carried
out such that the heater lamp is turned on only when the toner
image fixed on the recording sheet 130 exceeds the periphery of the
heat roller and when the surface temperature of the heat roller
starts to fall below the set temperature. In this case, the surface
temperature of the heat roller does not reach the set temperature
immediately, and therefore fixing from the point past the periphery
of the heat roller to the rear end of the recording sheet 130 is
carried out with the surface temperature of the heat roller below
the set temperature. As a result, a toner image cannot be fixed
uniformly on the recording sheet 130 from the front end to the rear
end.
SUMMARY OF THE INVENTION
The present invention offers a solution to the above-mentioned
problems, and accordingly it is an object of the present invention
to provide an image forming apparatus capable of suppressing
generation of ozone and thus the environmental toxicity to minimum,
and capable of realizing a desirable transfer characteristic in
accordance with a type of a recording sheet, which has been
transported, so as to prevent lowering of printing quality.
In order to achieve the above object, an image forming apparatus in
accordance with the present invention includes a latent image
holding section for holding color-separated image information as an
electrostatic latent image; a plurality of developing sections for
making the electrostatic latent image held by the latent image
holding section visible color by color; an intermediate transfer
medium to which a visualized image of each color visualized on a
surface of the latent image holding section is successively
transferred upon contact with the latent image holding section; a
transfer section, which is separable and contactable with respect
to the intermediate transfer medium; and a voltage applying section
for applying a predetermined voltage to the transfer section,
wherein the transfer section carries out both transfers of (A)
between the latent image holding section and the intermediate
transfer section and (B) between the intermediate transfer medium
and the recording medium in accordance with the voltage applied
from the voltage applying section.
With this arrangement, the electrostatic latent image formed on the
latent image holding section is made visible by the developer of
corresponding color. A plurality of visualized images obtained by
the plurality of developing sections are overlapped with one
another on the intermediate transfer medium, and thereafter are
transferred onto the recording medium from the intermediate
transfer medium by the application of a voltage from the voltage
applying section to the transfer section.
Here, the transfer section independently carries out both transfers
of (A) a first transfer for transferring the visualized images
formed on the latent image holding section to the intermediate
transfer section and (B) a second transfer for transferring the
visualized image overlapped on the intermediate transfer medium to
the recording medium. Therefore, compared with the conventional
case where the first transfer and the second transfer are carried
out by separate transfer sections, it is ensured that less ozone is
generated.
Namely, when the transfer section happens to be employing, for
example, corona discharge, due to the fact that the number of
transfer section is reduced, the ozone generated is also reduced.
On the other hand, when the transfer section is composed of, for
example, a contact roller, no ozone, originating from the transfer
section, is generated.
Therefore, with the described arrangement, it is ensured that the
ozone, which is toxic to the human body, generated from the whole
device is reduced, thus realizing an image forming apparatus which
is environmentally friendly.
Further, with the described arrangement, compared with the case
where the first transfer and the second transfer are carried out by
separate transfer sections, the number of transfer section is
reduced, and accordingly it is not required to provide members,
such as power source, corresponding to the transfer sections. As a
result, the number of components of the device is reduced, thus
realizing a compact device.
For a fuller understanding of the nature and advantages of the
invention, reference should be made to the ensuing detailed
description taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an explanatory drawing schematically showing an image
forming apparatus in accordance with the present invention.
FIG. 2 is a cross sectional view showing a schematic arrangement of
the image forming apparatus.
FIG. 3(a) and FIG. 3(b) are cross sectional views showing a contact
between an inner surface of a photoreceptor and a suspension roller
of the image forming apparatus in the case where a width of the
photoreceptor and a width of the suspension roller are
substantially the same.
FIG. 4(a) and FIG. 4(b) are cross sectional views showing a contact
between the inner surface of the photoreceptor and the suspension
roller of the image forming apparatus in the case where the width
of the photoreceptor and the width of the suspension roller are
substantially the same.
FIG. 5(a) and FIG. 5(b) are cross sectional views showing a contact
between the inner surface of the photoreceptor and the suspension
roller of the image forming apparatus in the case where the width
of the photoreceptor is larger than the width of the suspension
roller.
FIG. 6 is a cross sectional view showing a detailed arrangement of
a developing section of the image forming apparatus.
FIG. 7 is a cross sectional view showing a detailed arrangement of
a developing section of the image forming apparatus.
FIG. 8 is an enlarged cross sectional view of the developing
section.
FIG. 9 is a cross sectional view showing a detailed structure of a
separating-contacting mechanism of the photoreceptor of the image
forming apparatus.
FIG. 10 is a cross sectional view showing a detailed structure of
an intermediate transfer medium provided in the image forming
apparatus.
FIG. 11 is a block diagram showing a control by a CPU provided in
the image forming apparatus.
FIG. 12 is a timing chart showing one example of operations of
components of the image forming apparatus.
FIG. 13 is a timing chart showing another example of operations of
components of the image forming apparatus.
FIG. 14 is an explanatory drawing showing a detailed structure of a
sheet type detecting section of the image forming apparatus.
FIG. 15 is a graph showing a relationship between wavelength of
light emitted from a light emitting element of the sheet type
detecting section and transmittance for various types of recording
sheet.
FIG. 16 is a graph showing a relationship between basis weight of a
recording sheet and transmittance at a predetermined wavelength of
light.
FIG. 17 is a graph showing a relationship between basis weight of a
recording sheet and transmittance at a wavelength of light
different from that of FIG. 16.
FIG. 18 is a flowchart showing a control of the CPU in accordance
with a detection signal from the sheet type detecting section.
FIG. 19 is a graph showing a relationship between basis weight of a
recording sheet and transfer voltage applied in accordance with the
basis weight.
FIG. 20 is a graph showing a relationship between transfer voltage
and the amount of adhering toner with respect to a recording sheet
having a predetermined basis weight.
FIG. 21 is a graph showing a relationship between transfer voltage
and the amount of adhering toner with respect to a recording sheet
having a basis weight different from that of FIG. 20.
FIG. 22 is a graph showing a relationship between transfer voltage
and the amount of adhering toner with respect to a recording sheet
having a basis weight different from that of FIG. 20 and FIG.
21.
FIG. 23 is an explanatory drawing showing in logical representation
a change in temperature by a fixing temperature control and ON/OFF
timing of a heater lamp in the image forming apparatus.
FIG. 24 is a flowchart showing an operation of the fixing
temperature control.
FIG. 25 is an explanatory drawing showing an actual change in
fixing temperature in the image forming apparatus.
FIG. 26 is a cross sectional view showing a fixing roller as
another example of fixing means in the image forming apparatus.
FIG. 27 is a cross sectional view showing a schematic arrangement
of a conventional image forming apparatus.
FIG. 28 is an explanatory drawing showing a normal change in fixing
temperature when the heater lamp is turned on.
FIG. 29 is an explanatory drawing showing a change in fixing
ability with time.
FIG. 30 is an explanatory drawing showing an actual change in
fixing temperature in the conventional image forming apparatus.
DESCRIPTION OF THE EMBODIMENTS
The following will describe one embodiment of an image forming
apparatus in accordance with the present invention referring to
FIG. 1 through FIG. 26.
As shown in FIG. 1 and FIG. 2, a color copying machine (referred to
simply as "copying machine" hereinafter) as an image forming
apparatus in accordance with the present embodiment includes an
exposing section 1, a developing section 2, a transfer section 3, a
sheet-feeding section 4, and a fixing section 5. FIG. 1 is a
schematic drawing showing main components of the copying machine of
FIG. 2.
The exposing section 1 projects a laser beam in accordance with a
document image onto an outer surface of a photoreceptor 7
(described later) to form an electrostatic latent image thereon.
The developing section 2 makes the electrostatic latent image
visible using toner (developer). The transfer section 3 carries out
a so-called (1) first transfer in which a toner image formed on the
photoreceptor 7 is transferred onto an intermediate transfer medium
22 (described later) color by color and (2) second transfer in
which a color toner image formed on the intermediate transfer
medium 22 is transferred onto a recording sheet 6. The
sheet-feeding section 4 stocks recording sheet 6 (recording medium)
to which a color toner image is transferred and supplies the
recording sheet 6 to the transfer section 3. The fixing section 5
fuses and fixes the toner image transferred on the recording sheet
6. The following describes detailed arrangements of the above
sections in the order they were introduced.
The exposing section 1 is composed of a laser scanning unit 1a
which emits a laser beam in accordance with a document image and a
mirror 1b which reflects and guides the laser beam to the
photoreceptor 7. The laser beam emitted from the laser scanning
unit 1a is projected on the outer surface of the photoreceptor 7
between a charger 9 and a developer tank 13 (both mentioned later),
and this exposes the outer surface of the photoreceptor 7 and an
electrostatic latent image is formed on the photoreceptor 7.
The developing section 2 is provided with the photoreceptor 7
(latent image holding device) which is pressed against the
intermediate transfer medium 22. The photoreceptor 7 of the present
embodiment is a photoreceptive belt made from an OPC (organic
Photoconductive Conductor) film, and is suspended by two suspension
rollers 8.
As shown in FIG. 3(a) and FIG. 4(a), belt width P of the
photoreceptor 7 is made substantially the same as drum width Q of
the intermediate transfer medium 22. This is for the following
reason.
In general, the inner surface of the photoreceptor 7 is provided
with bead sections 7a. The bead sections 7a are brought into
contact with step-difference sections 8a provided on the both ends
in the axis direction of each of the suspension rollers 8, or
alternatively, as shown in FIG. 4(b), engaged with grooves 8b. The
bead sections 7a are regulated by the step-difference sections 8a
or by the grooves 8b and wobbling of the photoreceptor 7 is
minimized.
When the belt width P of the photoreceptor 7 is larger than the
drum width Q of the intermediate transfer medium 22, as shown in
FIG. 5(a), the bead sections 7a are more likely to be separated
from, for example, the step-difference sections 8a by the pressing
of the intermediate transfer medium 22, as shown in FIG. 5(b), and
this causes the photoreceptor 7 to wobble.
By providing the photoreceptor 7 in such a manner that the belt
width P is substantially equal to the drum width Q, the bead
sections 7 a are prevented from separating from the step-difference
sections 8a or from the grooves 8b, and wobbling of the
photoreceptor 7 in the axis direction is minimized.
Note that, in the arrangement of FIG. 3(a) and FIG. 3(b),
considering the margin required for placing the suspension rollers
8 on the inner surface of the photoreceptor 7, a clearance is
provided between the step-difference sections 8a and the bead
sections 7a in the axis direction of the suspension rollers 8.
Thus, even though the belt width P and the drum width Q are
substantially equal, slight wobbling of the photoreceptor 7 still
occurs. On the other hand, in the arrangement of FIG. 4(a) and FIG.
4(b), since the bead sections 7a are engaged with the grooves 8b,
the wobbling of the photoreceptor 7 in the axis direction is
substantially completely prevented. Therefore, in practice, the
arrangement of FIG. 4(a) and FIG. 4(b) is more preferable than that
of FIG. 3(a) and FIG. 3(b). Nevertheless, the photoreceptor 7
wobbles less often in the arrangement of FIG. 3(a) and FIG. 3(b)
compared with the arrangement of FIG. 5(a) and FIG. 5(b).
As shown in FIG. 1 and FIG. 2, around the photoreceptor 7 are
provided the charger 9, developer tanks 10 to 13 (developing
means), a density sensor (TMA: Toner Mass Area Sensor) 14, a
surface stabilizer 15, a cleaning device 16, and a discharge lamp
17.
The charger 9 charges the outer surface of the photoreceptor 7, and
in the present embodiment, is composed of a static charger with a
grid. Alternatively, the charger 9 may be composed of a static
brush or static roller, etc.
The developer tanks 10 to 13 store toners of yellow (Y), magenta
(M), cyan (C), and black (BK), respectively, and develop the
electrostatic latent image formed on the photoreceptor 7 with
respect to each color so as to form a toner image of each color on
the photoreceptor 7. Note that, in the present embodiment, the
toners of the above colors are negatively charged; however, the
principle of the present embodiment can also be applicable to
positively charged toner. Also, the developer tanks 10 to 13 of the
present embodiment are composed of a developer-hopper section which
is an integral unit of a developer section and a hopper
section.
The density sensor 14 detects the density of a toner layer on the
outer surface of the photoreceptor 7, and in the case where the
density is lower than the usual, tells an operator through, for
example, a display section (not shown) to replenish the toner of a
color displayed on the display section to the developer tank. The
surface stabilizer 15 is composed of at least one of a PTC
(Pre-Transfer Charger) and PTL (Pre-Transfer Lamp) and stabilizes
the charge on the outer surface of the photoreceptor 7 to increase
the transfer efficiency in the first transfer (described
later).
The cleaning device 16 removes waste toner which was not
transferred to the intermediate transfer medium 22 and is remaining
on the photoreceptor 7, and also removes waste toner adhering on a
transfer roller 23 (mentioned later). The cleaning device 16 of the
present embodiment is of a blade type, but may also be composed of
an electrostatic brush alternatively.
The discharge lamp 17 discharges the outer surface of the
photoreceptor 7. It is preferable that the electric potential on
the outer surface of the photoreceptor 7 is set to 0 V by the
effect of the discharge lamp 17. However, removal of charge needs
not to be so complete.
In the described photoreceptor 7, the process of charging,
exposure, development, and transfer is repeated for each color.
Thus, a toner image of one color is transferred onto the
intermediate transfer medium 22 from the photoreceptor 7 per one
rotation of the intermediate transfer medium 22, and a color toner
image is obtained by at the maximum of four rotations of the
intermediate transfer medium 22.
As shown in FIG. 6 and FIG. 7, on the side of the developer tanks
10 to 13 opposite to the photoreceptor 7, there are provided
developer tank pressing cams 10a to 13a, respectively, for pressing
the developer tanks 10 to 13 against the photoreceptor 7, the
developer tank pressing cams 10a to 13a being always in contact
with the developer tanks 10 to 13, respectively. The driving shafts
of the developer tank pressing cams 10a to 13a are all connected to
a single driving source. The developer tanks 10 to 13 are also
pressed against the corresponding developer tank pressing cams 10a
to 13a by retaining means such as springs 10e to 13e.
The following describes a separating-contacting operation of the
developer tanks 10 to 13 with respect to the photoreceptor 7 by the
rotation of the developer tank pressing cams 10a to 13a. Note that,
the following explanations are based on, for example, the structure
of the developer tank 12 filled with cyan toner and the
separating-contacting operation of the developer tank 12 with
respect to the photoreceptor 7. The other developer tanks 10, 11,
and 13 are the same as the developer tank 12, and therefore
explanations thereof are omitted here.
As shown in FIG. 8, the developer tank 12 is provided with a
development roller 12b and driving gears 12c.sub.1 to 12c.sub.3
which are in mesh with one another. The development roller 12b is
integrally and coaxially provided with the driving gear 12c.sub.3.
The driving gear 12c.sub.1 is provided in such a manner that it can
be brought into mesh with a driving gear 12d which is provided on
the copying machine main body. In FIG. 8, the driving gear 12d is
rotating counterclockwise. The developer tank pressing cam 12a is
provided such that the developer tank 12 is pressed against the
photoreceptor 7 by the rotation of the developer tank pressing cam
12a when developing, and that the development roller 12b is
separated from the photoreceptor 7 by a predetermined distance when
not developing.
In this arrangement, when the developer tank 12 is pressed against
the photoreceptor 7 by the counterclockwise rotation of the
developer tank pressing cam 12a, the driving gear 12c.sub.1 is
brought into mesh with the driving gear 12d, and the development
roller 12b comes into contact with the outer surface of the
photoreceptor 7. This transmits the rotational force of the driving
gear 12d to the development roller 12b via the driving gears
12c.sub.1 to 12c.sub.3, and the development roller 12b rotates
clockwise in FIG. 8. As a result, the cyan toner is released from
the developer tank 12, and a toner image of cyan is formed on the
photoreceptor 7.
Then, as the developer tank pressing cam 12a rotates further, by
the retaining force of the spring 12e, the developer tank 12
separates from the photoreceptor 7 while being in contact with the
developer tank pressing cam 12a. This releases the contact between
the development roller 12b and the photoreceptor 7, and the
development of cyan is finished. Here, the development roller 12b
is held with a constant distance from the photoreceptor 7 all the
time until the next round of development is started.
Therefore, even when the developer tank 12 is shuddered by the
rotation of the development roller 12b and the driving gears
12c.sub.1 to 12c.sub.3, because the development roller 12b and the
photoreceptor 7 are separated from each other with a predetermined
distance when development is not being carried out, deterioration
of image quality due to shudder does not occur and a high quality
image is obtained.
Note that, it is not necessarily the case that the order of
development by the developer tanks 10 to 13 starts from the
developer tank 12. It may start from, for example, development of
black by the developer tank 13. FIG. 6 through FIG. 8 illustrate
such a case in the pressing mechanism of the developer tanks 10 to
13.
As shown in FIG. 2, the developing section 2 is provided with a
belt separating-contacting mechanism 18. The belt
separating-contacting mechanism 18 is composed of (i) a tension
plate 19 which is rotatably movable in the B-C directions in FIG. 2
round the support of a rotation shaft 19a in the vicinity of one of
the suspension rollers 8, (ii) a tension roller 20 attached to the
tension plate 19, contacting the inner surface of the photoreceptor
7, and (iii) a release shaft 21 which can be brought into contact
with the outer surface of the photoreceptor 7.
Therefore, as shown in FIG. 9, when the tension plate 19 is
rotatably moved in the B direction in FIG. 9, the contact between
the photoreceptor 7 and the intermediate transfer medium 22 is
released as the release shaft 21 is brought into contact with the
outer surface of the photoreceptor 7. This allows the photoreceptor
7, whose life has ended for example, to be taken out of the copying
machine main body and replaced with a new one. To install the
photoreceptor 7 in the copying machine main body, the photoreceptor
7 is inserted into a predetermined position with the tension plate
19 rotatably moved in the B direction, and then the tension plate
19 is rotatably moved in the C direction so as to allow the outer
surface of the photoreceptor 7 to contact with the intermediate
transfer medium 22, thus installing the photoreceptor 7 in the
copying machine without damaging the outer surface of the
photoreceptor 7.
As shown in FIG. 1 and FIG. 2, the transfer section 3 includes the
intermediate transfer medium 22 having a drum shape, and the toner
image formed on the outer surface of the photoreceptor 7 is
transferred onto the intermediate transfer medium 22. The
intermediate transfer medium 22 is composed of, as shown in FIG.
10, a metal drum 22a having a cylindrical shape and a
semiconductive resin film 22b. The semiconductive resin film 22b is
integrally formed around the outer surface of the metal drum 22a by
heat-shrinkage, and is made of a material such as polyimide, nylon,
and fluorine.
When the metal drum 22a and the semiconductive resin film 22b are
integrally formed in this manner, the intermediate transfer medium
22 can be used for the same duration as the machine life of the
copying machine main body. Also, since the intermediate transfer
medium 22 is a drum unit, compared with the case where the
intermediate transfer medium is composed of a belt and a plurality
of suspension rollers, the number of components can be reduced,
thus reducing the overall costs.
Also, when a belt intermediate transfer medium is adopted, the
overlapping accuracy of toner images from the photoreceptor to the
intermediate transfer medium is lowered by the wobbling of the
belt. However, in the present embodiment, since the intermediate
transfer medium 22 having a drum shape is adopted, wobbling does
not occur, and the lowering of the overlapping accuracy of toner
images as caused by wobbling is prevented.
The volume resistivity of the semiconductive resin film 22b is set
in a range of 10.sup.6 to 10.sup.12 .OMEGA..multidot.cm. When the
volume resistivity is smaller than 10.sup.6 .OMEGA..multidot.cm,
the voltage applied to the intermediate transfer medium 22 is given
off via the metal drum 22a, and does not remain on the intermediate
transfer medium 22. When the volume resistivity is larger than
10.sup.12 .OMEGA..multidot.cm, the voltage applied to the
intermediate transfer medium 22 is held in excess and sufficient
discharge cannot be carried out even when discharge is required. As
a result, the potential of the semiconductive resin film 22b is
increased further, raising the possibility of leaking between the
intermediate transfer medium 22 and the photoreceptor 7 upon
contact. Therefore, in the present embodiment, the volume
resistivity of the semiconductive resin film 22b is set in the
above range, ensuring that a predetermined voltage is maintained
and sufficient discharge is carried out when discharge is required.
As a result, first through fourth transfers (described later) are
carried out with certainty.
In addition to the intermediate transfer medium 22, the transfer
section 3 further includes the transfer roller 23 (transfer
device), a sheet removal charger 24, a pre-transfer discharger 25,
and a cleaning device 26.
The transfer roller 23 presses the recording sheet 6, which has
been transported to the transfer section 3, against the
intermediate transfer medium 22. The transfer roller 23 is
supported by a supporting member 27, and is retained all the time
in a direction towards the intermediate transfer medium 22 by a
transfer roller pressing spring 28 attached to the supporting
member 27. The pressing and separating operation of the transfer
roller 23 with respect to the intermediate transfer medium 22 is
controlled by an ON/OFF operation of a transfer roller separating
solenoid 29 attached to the supporting member 27.
The transfer roller 23 is connected to a power source 23a (voltage
applying device) and a predetermined voltage is applied to the
transfer roller 23 from the power source 23a. Note that, details of
voltage application of the power source 23a to the transfer roller
23 will be described later.
The sheet removal charger 24 is supported by the supporting member
27 as with the transfer roller 23, and is operated in
synchronization with the separating-contacting operation of the
transfer roller 23 with respect to the intermediate transfer medium
22. The sheet removal sheet 24 is connected to a power source 24a.
Thus, after transferring of a color toner image from the
intermediate transfer medium 22 to the recording sheet 6 is
finished, by application of a predetermined voltage from the power
source 24a, the recording sheet 6 is removed electrostatically from
the intermediate transfer medium 22. The recording sheet 6 removed
is transported to the fixing section 5 by being guided by a
transport guide 30.
Note that, the operation of the sheet removal charger 24 is not
necessarily required to be in synchronization with the
separating-contacting operation of the transfer roller 23.
Alternatively, the sheet removal charger 24 may be fixably provided
in the vicinity of the intermediate transfer medium 22 without the
support of the supporting member 27.
The pre-transfer discharger 25 removes charge on the surface of the
intermediate transfer medium 22 before the toner image of each
color is transferred to the intermediate transfer medium 22 from
the photoreceptor 7, and is composed of, for example, PTC. This
reduces the amount of charge on toner on the surface of the
intermediate transfer medium 22 and increases the transfer
efficiency in the second transfer (described later) . The cleaning
device 26 removes toner adhered on the surface of the intermediate
transfer medium 22.
Note that, in the present embodiment, the cleaning device 26 is not
necessarily required and can be excluded. When the cleaning device
26 is provided, it is ensured that the surface of the intermediate
transfer medium 22 is cleaned by the cleaning device 26 per
predetermined rotations of the intermediate transfer medium 22.
On the both ends in the axis direction of the intermediate transfer
medium 22 are provided flanges made of insulator. The intermediate
transfer medium 22 is mounted on the frame of the main body via
bearings attached to the flanges. Thus, the intermediate transfer
medium 22 is rotatably fixed while being insulated from the main
body frame, and adverse electric effects from surrounding devices
are eliminated, thus maintaining the surface of the intermediate
transfer medium 22 electrically stable.
The following will describe in detail voltage application of the
power source 23a to the transfer roller 23. The power source 23a
applies, as shown in Table 1, different voltages to the transfer
roller 23 in a pre-processing stage (first stage), a transfer stage
(second stage), and a post-processing stage (third stage).
TABLE 1 Pre- Post- Processing Transfer Processing Stage . . . Stage
Stage . . . State of contacting separating contacting contacting
separating Transfer Roller With Respect to Inter- mediate Transfer
Medium Voltage +50 V to 0 V +100 V to -500 V to 0 V Applied to +500
V +2000 V +5000 V Transfer -50 V to Roller -500 V
Here, the pre-processing stage refers to a period between turning
on of the copying machine and the start of development of a first
color (for example, yellow). In this stage, the transfer roller
separating solenoid 29 is turned off, and the transfer roller 23 is
brought into contact with the intermediate transfer medium 22, and
the power source 23a applies a first voltage, for example in a
range of +50 V to +500 V, having the opposite polarity to that of
the toner, to the transfer roller 23. The first voltage in this
range is sufficient for first transfer. As a result, a charge
required for first transfer is sufficiently maintained on the
surface of the intermediate transfer medium 22, setting a condition
for the first transfer of the toner image of each color from the
photoreceptor 7 to the intermediate transfer medium 22.
When the application of the predetermined voltage by the power
source 23a is finished, the transfer roller separating solenoid 29
is turned on, and the transfer roller 23 is separated from the
intermediate transfer medium 22. The transfer roller 23 is kept
separated from the intermediate transfer medium 22 while
maintaining the applied voltage to the transfer roller 23 at 0 V
until transfer of the toner images of all four colors from the
photoreceptor 7 to the intermediate transfer medium 22 is finished.
This prevents the toner image transferred to the intermediate
transfer medium 22 from being disturbed by the transfer roller
23.
The transfer stage refers to the stage of second transfer, in which
the color toner image of four colors formed on the intermediate
transfer medium 22 is transferred to the recording sheet 6 at once.
Thus, in this stage, the transfer roller separating solenoid 29 is
turned off again at a timing when the front end of the color toner
image formed on the intermediate transfer medium 22 coincides with
the front end of the recording sheet 6, and the transfer roller 23
is brought into contact with the intermediate transfer medium 22.
Here, a second voltage, for example, a voltage in a range of +100 V
to +2000 V, higher than the voltage applied in the pre-processing
stage is applied to the transfer roller 23 by the power source 23a.
The second voltage in this range is sufficient for second
transfer.
As a result, the color toner image is transferred to a
predetermined position on the recording sheet 6, and the second
transfer from the intermediate transfer medium 22 to the recording
sheet 6 is smoothly carried out. Note that, in the transfer stage,
the transfer roller separating solenoid 29 is not turned on even
when the second transfer is finished, and the transfer roller 23 is
kept contacted with the intermediate transfer medium 22.
The post-processing stage refers to a stage in which a third
transfer and a fourth transfer are carried out: the third transfer
for transferring toner adhering on the transfer roller 23 back to
the intermediate transfer medium 22 after second transfer; and the
fourth transfer for transferring toner adhering on the intermediate
transfer medium 22 back to the photoreceptor 7 after the third
transfer. Namely, the post-processing stage cleans the surfaces of
the intermediate transfer medium 22 and the transfer roller 23, and
therefore is a preparing stage for the next copying operation.
In the third transfer, continuing from the second transfer, the
transfer roller separating solenoid 29 remains off, and the
transfer roller 23 remains contacting the intermediate transfer
medium 22. And, a third voltage, for example, a voltage in a range
of -500 V to +1500 V, lower than the second voltage is applied to
the transfer roller 23 by the power source 23a. The third voltage
in this range is sufficient for third transfer.
Here, when a voltage of, for example, +2000 V is applied to the
transfer roller 23 in the second transfer, a voltage of, for
example, +1500 V is applied to the transfer roller 23 in the third
transfer. That is, a voltage applied in the third transfer is more
negative than a voltage applied in the second transfer. This is
indeed the same as applying a negative voltage. As a result, it is
ensured that the toner adhering on the transfer roller 23 is
returned to the intermediate transfer medium 22, thus ensuring that
the rear surface of the recording sheet 6 is not contaminated by
the contaminants on the transfer roller 23.
Meanwhile, in the fourth transfer, a fourth voltage still lower
than the third voltage, for example, a voltage in a range of -50 V
to -500 V is applied to the transfer roller 23 by the power source
23a while the transfer roller 23 remains contacting the
intermediate transfer medium 22. The fourth voltage in this range
is sufficient for fourth transfer. As a result, it is ensured that
the toner adhering on the intermediate transfer medium 22 is
returned to the photoreceptor 7, and the surface of the
intermediate transfer medium 22 is cleaned. Also, because the third
voltage and the fourth voltage are different, the toner on the
intermediate transfer medium 22 does not adhere on the transfer
roller 23, thus cleaning both the transfer roller 23 and the
intermediate transfer medium 22. When the fourth transfer is
finished, the transfer roller separating solenoid 29 is turned on,
and the transfer roller 23 is separated from the intermediate
transfer medium 22, finishing the preparation for the next image
forming process.
Note that, in the described transfer stages, a wide range of
voltages are applied to the transfer roller 23. This is for
consideration of the thickness of the recording sheet 6,
diminishing of the charge with time, and other variables involved.
Namely, as the recording sheet 6 becomes thicker or the charge
diminishes abruptly, a higher voltage is required. The voltage
applied in each of the above transfer stages is controlled by a CPU
47 (mentioned later).
The sheet feeding section 4 is provided with a sheet feeding
cassette 31 for stocking the recording sheet 6 of a predetermined
size and a manual sheet feeding section 32. The sheet feeding
section 31 is provided below the manual sheet feeding section 32
and is detachable with respect to the copying machine. The
recording sheet 6 stocked in the sheet feeding section 31 is
transported one by one from the top of the sheet feeding cassette
31 by a pickup roller 33 towards the transfer section 3. In
contrast, the recording sheet 6 manually supplied one by one to the
manual sheet feeding section 32 is transported to the transfer
section 3 by a manual roller 34.
The sheet feeding section is also provided with a sheet presence
detecting sensor 35, a sheet type detecting section 36 (sheet type
detecting means), and aligning rollers 37. These are provided in
this order along the transport direction of the recording sheet 6
from the manual roller 34 to the transfer section 3.
The sheet presence detecting sensor 35 detects the presence or
absence of the recording sheet 6 transported from the sheet feeding
section 4 to the transfer section 3. The sheet type detecting
section 36 detects the sheet type, such as transparency, thickness,
color, and length, of the recording sheet 6, which are used as
criteria for deciding whether the recording sheet 6 is a
transparent film used in an OHP (Overhead Projector) or a
non-transparent film. Note that, in the following, the transparent
film will be referred to as an OHP film.
The sheet type detecting section 36 is provided between the manual
sheet feeding section 32 and the transfer section 3. This ensures
early detection of the sheet type of the recording sheet 6,
allowing easy control of the transfer voltage applied to the
transfer roller 23 and of the fixing temperature in the fixing
section 5 in the following process. The sheet type detecting
section 36 will be described later in more detail.
In the case of automatic feeding, the CPU 47 (mentioned later)
detects the type of the sheet feeding cassette 31 storing the
recording sheet 6 of a predetermined size so as to detect the
length of the recording sheet 6 based on the type of the sheet
feeding cassette 31 thus detected. In this case, the length of the
recording sheet 6 is detected before the detection by the sheet
type detecting section 36, thus allowing the controls in the
following processes to be carried out with more ease.
The aligning rollers 37 temporarily stop the recording sheet 6
being transported so as to transport the recording sheet 6 to the
transfer section 3 at a predetermined timing. The predetermined
timing is the instance where the front end of the color toner image
on the intermediate transfer medium 22 coincides with the front end
of the recording sheet 6 at a transfer position of the color toner
image from the intermediate transfer medium 22 to the recording
sheet 6.
The fixing section 5 includes a transport guide 38, fixing rollers
39a and 39b, a heat roller 40, a fixing temperature sensor 41, and
a sheet sensor 42.
The transport guide 38 guides the recording sheet 6 transported
from the transfer section 3 to a region between the fixing rollers
39a and 39b. The fixing rollers 39a and 39b fuse the toner image
with a predetermined temperature and pressure so as to fix the
toner image on the recording sheet 6. The heat roller 40 is
provided with a heater lamp 40a (heating device), and the
temperature of the surface of the heat roller 40 is set by the
ON/OFF operation of the heater lamp 40a. The fixing roller 39a and
the heat roller 40 are suspended by a fixing belt 43 (fixing
device). The fixing temperature sensor 41 detects the temperature
on the surface of the heat roller 40. The sheet sensor 42 detects
the presence or absence of the recording sheet 6 discharged from
the fixing rollers 39a and 39b. Note that, details of a fixing
temperature control will be described later.
On the downstream side of the sheet sensor 42 in the transport
direction of the recording sheet 6 are provided a discharge roller
44 and a discharge tray 45. The discharge roller 44 discharges the
recording sheet 6 which has been fixed to the discharge tray 45,
and the discharge tray 45 receives the recording sheet 6 thus
discharged.
As shown in FIG. 11, the copying machine main body is provided with
an environment temperature-humidity measuring sensor 46 and the CPU
(Central Processing Unit) 47. The environment temperature-humidity
measuring sensor 46 measures the temperature and humidity of the
environment in which the copying machine is set.
The CPU 47 (control means, fixing temperature control means)
controls various parameters in accordance with output signals from
the sheet type detecting section 36, the fixing temperature sensor
41, and the environment temperature-humidity measuring sensor 46.
The various parameters include, for example, the voltage of the
power source 23a applied to the transfer roller 23, the voltage of
the power source 24a applied to the sheet removal charger 24, an
image bias voltage, a voltage for charging the photoreceptor 7, the
temperature of the heater lamp 40a, the rotational speed of the
driving motor 48 for driving the fixing rollers, and the amount of
light emitted by a light emitting element 51 (mentioned later) of
the sheet type detecting section 36, etc.
The temperature control of the heater lamp 40a is carried out by a
fixing system temperature control circuit 49 in accordance with a
control signal from the CPU 47. The rotational speed of the driving
motor 48 is controlled by a motor rotational speed control circuit
50 in accordance with a control signal from the CPU 47.
The following describes the operation of the copying machine having
the described arrangement referring to FIG. 1, FIG. 2, and FIG. 12.
Note that, in the following, a contact between the photoreceptor 7
and the intermediate transfer medium 22 will be referred to as a
transfer position X, and a contact between the intermediate
transfer medium 22 and the transfer roller 23 will be referred to
as a transfer position Y. Also, the explanation will be given
through the case where the length of the recording sheet 6 is
longer than the distance between the transfer position Y and the
transfer position X on the surface of the intermediate transfer
medium 22 in the rotation direction of the intermediate transfer
medium 22.
First, when the copying machine is turned on, the photoreceptor 7
and the intermediate transfer medium 22 start rotating. Before a
toner image is formed on the photoreceptor 7, the waste toner
remaining on the outer surface of the photoreceptor 7 is removed by
the cleaning device 16, and thereafter the outer surface of the
photoreceptor 7 is discharged by the discharge lamp 17. Here, the
transfer roller separating solenoid 29 is turned off. As a result,
by the effect of the transfer roller contacting spring 28, the
transfer roller 23 attached to the supporting member 27 is brought
into contact with the intermediate transfer medium 22, and a first
voltage in a range of +50 V to +500 V is applied by the power
source 23a to the intermediate transfer medium 22 via the transfer
roller 23 until the development of a first color (for example,
yellow) is started, and the first voltage is held on the
intermediate transfer medium 22 (pre-processing stage).
Then, after the outer surface of the photoreceptor 7 is charged by
the charger 9, the laser scanning unit 1a projects a laser beam in
accordance with a color image of yellow of a document image onto
the outer surface of the photoreceptor 7 via the mirror 1b so as to
expose and scan the photoreceptor 7. As a result, an electrostatic
latent image corresponding to the color image of yellow is formed
on the outer surface of the photoreceptor 7.
When the electrostatic image comes to a position to be developed by
the developer tank 10 as a result of the rotation of the
photoreceptor 7, the developer tank 10 is pressed against the
photoreceptor 7 by the developer tank pressing cam 10a, and the
development is carried out by the developer tank 10. As a result, a
toner image of yellow is formed on the photoreceptor 7. Thereafter,
the developer tank 10 is pushed back in a direction away from the
photoreceptor 7 by the effect of the spring 10e, and a
predetermined distance is maintained between the photoreceptor 7
and the developer tank 10. Note that, at the start of development
by the developer tank 10, the transfer roller separating solenoid
29 is turned on, and the transfer roller 23 is separated from the
intermediate transfer medium 22.
When the toner image comes to the transfer position X by the
rotation of the photoreceptor 7, because the first voltage having
the opposite polarity to that of the toner is held on the
intermediate transfer medium 22, the toner image is transferred to
the intermediate transfer medium 22 from the photoreceptor 7. Then,
the outer surface of the photoreceptor 7 is cleaned again by the
cleaning device 16 and is discharged by the discharge lamp 17. The
same process of charging, exposure, transfer, and discharge is also
carried out with respect to each of the other color images of
magenta, cyan, and black. Note that, during this, the transfer
roller 23 is kept separated from the intermediate transfer medium
22.
Therefore, by at the maximum of four rotations of the intermediate
transfer medium 22, a single full-color image is obtained on the
intermediate transfer medium 22. To obtain a monotone image, such
as a black-and-white image, from toner of an arbitrary color, only
one rotation of the intermediate transfer medium 22 is required.
After the toner images of all colors are transferred onto the
intermediate transfer medium 22, the color toner image composed of
the overlapping toner images is carried to the transfer position Y
in accordance with the rotation of the intermediate transfer medium
22.
Meanwhile, in accordance with the above image forming process, in
the case of automatic feeding, the recording sheet 6 is
sequentially sent out one by one from the top of the sheet feeding
cassette 31 by the pickup roller 33 to the aligning rollers 37. In
the case of manual feeding, the recording sheet 6 sent out one by
one from the manual sheet feeding section 32 and is transported to
the aligning rollers 37 by the manual roller 34. Note that, in
either case, the recording sheet 6 passes by the sheet presence
detecting sensor 35 and the sheet type detecting sensor 36 before
reaching the aligning rollers 37 so that the presence or absence of
the sheet is judged and the sheet type is detected. The aligning
rollers 37 temporarily stop the recording sheet 6 being transported
and then transport the recording sheet 6 to the transfer position Y
at such a timing that the front end of the color toner image on the
intermediate transfer medium 22 and the front end of the recording
sheet 6 coincide at the transfer position Y.
When the front end of the color toner image on the intermediate
transfer medium 22 and the front end of the recording sheet 6 reach
the transfer position Y, the transfer roller separating solenoid 29
is turned off, and the transfer roller 23 is pressed against the
intermediate transfer medium 22 via the recording sheet 6, and a
second voltage in a range of +100 V to +2000 V is applied by the
power source 23a. In this manner, by the application of the second
voltage having the opposite polarity to that of toner and having
larger absolute value than that of the first voltage, the color
toner image on the intermediate transfer medium 22 is transferred
(second transfer) to the recording sheet 6 (transfer stage).
The recording sheet 6 finished with second transfer is
electrostatically removed by the sheet removal charger 24, and is
transported to the region between the transfer rollers 39a and 39b
by being guided by the transport guides 30 and 38. The fixing belt
43 suspended by the transfer roller 39a and the heat roller 40 is
maintained at a predetermined temperature by the ON/OFF control of
the heater lamp 40a. Therefore, as the recording sheet 6 passes
through a region between the fixing belt 43 and the fixing roller
39b, the color toner image, which has not been fixed, is fixed on
the recording sheet 6 at a predetermined temperature and pressure.
Thereafter, the recording sheet 6 is discharged to the discharge
tray 45 by the discharge roller 44.
Meanwhile, because the transfer roller separating solenoid 29
remains off after the second transfer, the transfer roller 23
remains contacting the intermediate transfer medium 22. When the
second transfer is finished, the power source 23a applies the third
voltage in a range of -500 V to +1500 V to the transfer roller 23.
As a result, the toner adhered to the transfer roller 23 in the
second transfer is transferred (third transfer) to the intermediate
transfer medium 22 at the transfer position Y (post-processing
stage).
After the third transfer, the power source 23a applies the fourth
voltage in a range of -50 V to -500 V to the transfer roller 23. As
a result, the waste toner remaining on the intermediate transfer
medium 22 is transferred (fourth transfer) to the photoreceptor 7
at the transfer position X (post-processing stage).
When the fourth transfer is finished, the photoreceptor 7 and the
intermediate transfer medium 22 stop rotating, and the transfer
roller separating solenoid 29 is turned on, and the transfer roller
23 is separated from the intermediate transfer medium 22.
Note that, the first through fourth voltages are set in accordance
with the sheet type, etc., of the recording sheet 6 by the control
of the CPU 47.
As described, in the present embodiment, the first transfer and
second transfer are carried out by the transfer roller 23, not by a
transfer charger employing corona discharge, thus ensuring that
less ozone is generated compared with the conventional case.
Therefore, it is possible to provide a copying machine which causes
almost no harmful effect on the human body and is environmentally
friendly. Also, since the described transfers are carried out by a
single transfer roller 23 and a single power source 23a, the number
of components are much less than it had been required
conventionally, thereby realizing a small copying machine.
Further, the transfer roller 23 is brought into contact with the
intermediate transfer medium 22 only in the pre-processing stage,
transfer stage, and the post-processing stage, and is separated
from the intermediate transfer medium 22 in other times. This
prevents filming on the surface of the intermediate transfer medium
22 and also prevents a visualized toner image from being
transferred to the transfer roller 23 by pressure. As a result, the
toner image to be transferred to the recording sheet 6 is prevented
from being disturbed and the image quality is improved with
certainty. Note that, filming is a phenomenon in which the toner
sticks to the intermediate transfer medium 22 by being stretched
over at the contact between the transfer roller 23 and the
intermediate transfer medium 22 as a result of continuous contact
between these two members.
Also, as described, because the voltage applied to the transfer
roller 23 by the power source 23a is in accordance with the
pre-processing stage, transfer stage, and post-processing stage, it
is ensured that the transfer efficiency is improved in each of the
above stages.
Further, because the first voltage and second voltage have the
opposite polarity to that of the toner, and because the second
voltage is higher than the first voltage, it is ensured that the
second transfer is carried out when a transition is made from the
first transfer to the second transfer. Also, because the third
voltage and fourth voltage have the same polarity as that of toner,
or are voltages that are shifted from the side of the polarity of
the second voltage to the side of the polarity of the toner, and
because the fourth voltage is higher than the third voltage, it is
ensured that the toner is transferred in the order of the transfer
roller 23, the intermediate transfer medium 22, and the
photoreceptor 7.
Note that, in the post-processing stage, the third transfer and
fourth transfer may be carried out simultaneously. In such a case,
even though the third voltage and forth voltage are set to the same
voltage in a range of, for example, -50 V to -500 V, the third
transfer and fourth transfer are carried out appropriately. Also,
by carrying out the third transfer and fourth transfer
simultaneously, the processing time of the post-processing stage is
reduced, allowing a quick response to the next image forming
process. Note that, a mode in which the fourth transfer is carried
out after the third transfer and a mode in which the third transfer
and the fourth transfer are carried out simultaneously are
selectable.
Before the first transfer, and in the third transfer and fourth
transfer, the power source 23a applies a predetermined voltage to
the transfer roller 23 for a duration longer than one rotation of
the intermediate transfer medium 22. This ensures in the third
transfer that the toner remaining on the transfer roller 23 is
transferred back to the intermediate transfer medium 22. Also, in
the fourth transfer, it is ensured that the toner remaining on the
intermediate transfer medium 22 is transferred back to the
photoreceptor 7. As a result, in the subsequent first transfer, the
entire surface of the intermediate transfer medium 22 is uniformly
charged, thus substantially completely eliminating transfer
nonuniformity of the toner image on the intermediate transfer
medium 22. Therefore, with the described arrangement, the toner
image is transferred uniformly from the front end to the rear end
of the recording sheet 6. Note that, in the third transfer, it is
sufficient when the power source 23a applies a predetermined
voltage to the transfer roller 23 for a duration longer than one
rotation of the transfer roller 23.
Further, after the second transfer, the transfer roller 23 is not
separated from the intermediate transfer medium 22, and the third
transfer and fourth transfer are carried out successively, thus
simplifying the separating-contacting control of the transfer
roller 23.
As described, in the present embodiment, the transfer roller 23
carries out both transfers of (a) the transfer between the
photoreceptor 7 and the intermediate transfer medium 22 and (b) the
transfer between the intermediate transfer medium 22 and the
recording sheet 6.
After the fourth transfer, when the intermediate transfer medium 22
and the transfer roller 23 are kept contacted with each other, the
contact between these two members are deformed by the pressing
force exerted on one another. However, in the present embodiment,
because the transfer roller 23 is separated from the intermediate
transfer medium 22 after the fourth transfer, deformation of the
intermediate transfer medium 22 and the transfer roller 23 is
prevented.
Further, as described, by the separating-contacting operation of
the transfer roller 23 and by the application of a voltage by the
power source 23a, the waste toner is all transferred to the
photoreceptor 7 and remains only on the outer surface of the
photoreceptor 7. Therefore, it is not required to provide the
cleaning device 26 for cleaning the surface of the intermediate
transfer medium 22 and means to clean the surface of the transfer
roller 23. That is, the only cleaning device required in the
copying machine main body is the cleaning device 16 for cleaning
the photoreceptor 7. As a result, the number of components is
reduced, thus reducing the size of the copying machine and the
costs. Also, since only one cleaning device is provided, scattering
of toner in the copying machine is reduced.
Note that, in the present embodiment, as shown in FIG. 12, the
transfer roller 23 is pressed against the intermediate transfer
medium 22 and the power source 23a applies a predetermined second
voltage when the toner image of black is transferred to the
intermediate transfer medium 22 and the front end of the color
toner image on the intermediate transfer medium 22 reaches the
transfer position Y. However, as shown in FIG. 13, it is possible
alternatively to press the transfer roller 23 against the
intermediate transfer medium 22 and to apply the predetermined
second voltage by the power source 23a immediately after the
development of black is finished. This may be carried out when the
length of the recording sheet 6 is shorter than the distance
between the transfer position Y and the transfer position X on the
surface and in the rotation direction of the intermediate transfer
medium 22.
The following will describe a detailed structure of the sheet type
detecting section 36 of the present embodiment.
As shown in FIG. 14, the sheet type detecting section 36 optically
detects the type (transparency, thickness, length, color, etc.) of
the recording sheet 6 transported to the transfer section 3, and is
composed of the light emitting element 51, light receiving elements
52 and 53, an emittion light quantity control circuit 54, and a
received light quantity control circuit 55.
The light receiving element 52 is positioned such that the light
receiving element 52 can receive the light emitted from the light
emitting element 51 and transmitted through the recording sheet 6.
Thus, the light emitting element 51 and the light receiving element
52 constitute a transmissive sensor 56. The light receiving element
53 is positioned such that the light receiving element 53 can
receive the light emitted from the light emitting element 51 and
reflected off the upper surface of the recording sheet 6. Thus, the
light emitting element 51 and the light receiving element 53
constitute a reflective sensor 57. Namely, in the present
embodiment, the transmissive sensor 56 and the reflective sensor 57
are used in combination. Note that, the light receiving element 53
of the reflective sensor 57 is composed of, for example, a CCD
(Charge Coupled Device) capable of recognizing the color of the
recording sheet 6.
The emittion light quantity control circuit 54 controls the amount
of light emitted by the light emitting element 51, in accordance
with a control signal from the CPU 47. The received light quantity
control circuit 55 sends signals corresponding to the amount of
light received by the light receiving elements 52 and 53,
respectively, to the CPU 47.
FIG. 15 shows a relationship between wavelength and transmittance
of light emitted from the light emitting element 51. In FIG. 15,
the curve a.sub.1 connecting "{character pullout}", the curve
a.sub.2 connecting ".box-solid.", the curve a.sub.3 connecting
".tangle-solidup.", the curve a.sub.4 connecting ".times.", the
curve a.sub.5 connecting ".quadrature.", the curve a.sub.6
connecting ".circle-solid.", the curve a.sub.7 connecting
".DELTA.", the curve a.sub.8 connecting ".smallcircle.", the curve
a.sub.9 connecting "{character pullout}" represent the wavelength
vs. transmittance relationship when the recording sheet 6 is a
sheet of paper having a basis weight (mass per unit area) of 52
g/m.sup.2, 60 g/m.sup.2 (#1), 60 g/m.sup.2 (#2), 80 g/m.sup.2 (#1),
80 g/m.sup.2 (#2), 100 g/m.sup.2, 128 g/m.sup.2, 184 g/m.sup.2, and
an envelope, respectively. It can be seen from FIG. 15 that the
transmittance of light is different depending on the basis weight
of the recording sheet 6 and the wavelength of the light.
Note that, a common domestic envelope is made by laminating two to
four sheets of paper each having a basis weight of 50 g/m.sup.2 to
60 g/m.sup.2, and a common air mail envelope is made by laminating
7 or so sheets of paper each having a basis weight in the range of
50 g/m.sup.2 to 60 g/m.sup.2. Therefore, a domestic envelope is
equivalent of a sheet of paper having a basis weight of 100
g/m.sup.2 to 240 g/m.sup.2, and an air mail envelope is equivalent
of a sheet of paper having a basis weight of 350 g/m.sup.2 to 420
g/m.sup.2. The envelope used in the present embodiment is
equivalent of a sheet of paper having a basis weight of 240
g/m.sup.2.
As shown in FIG. 15, when the wavelength of light is in a range of
400 nm to 760 nm, there is a case where the same transmittance is
obtained at a predetermined wavelength even when the basis weight
of the recording sheet 6 is different. Thus, when the wavelength of
light is in this range, it is impossible to detect the basis weight
of the recording sheet 6 by detecting the transmittance, namely the
thickness of the recording sheet 6 cannot be detected. In contrast,
when the wavelength of light is 800 nm or longer, the transmittance
and the wavelength correspond to each other one to one. Therefore,
in this case, it is possible to detect the basis weight of the
recording sheet 6 by detecting the transmittance. This is clear
from the graphs of FIG. 16 and FIG. 17.
FIG. 16 shows a relationship between the basis weight of the
recording sheet 6 and transmittance when the wavelength of the
light emitted from the light emitting element 51 is 400 nm. As
shown in FIG. 16, there is a case where the same transmittance is
obtained at two or more types of the recording sheet 6 having
different basis weights. Thus, in some cases, the basis weight of
the recording sheet 6 cannot be decided depending on the
transmittance. In contrast, FIG. 17 shows a relationship between
the basis weight of the recording sheet 6 and transmittance when
the wavelength of light is 840 nm. In this case, the transmittance
and the wavelength correspond to each other one to one.
Therefore, with a wavelength of light of 800 nm or longer, by
measuring the transmittance of the light transmitted through the
recording sheet 6, it is possible to discriminate from one another
the types of the recording sheet 6 having a basis weight in a range
of 40 g/m.sup.2 to 300 g/m.sup.2.
Specifically, from the graph of FIG. 17, the recording sheet 6 is
decided to have a basis weight in a range of 50 g/m.sup.2 to 100
g/m.sup.2 when the transmittance is not less than 18 percent, and
to have a basis weight in a range of 100 g/m.sup.2 to 150 g/m.sup.2
when the transmittance is in a range of not less than 16 percent to
less than 18 percent. Also, from the slope of the graph of FIG. 17,
it is possible decide that the recording sheet 6 has a basis weight
in a range of 40 g/m.sup.2 to 100 g/m.sup.2 when the transmittance
is not less than 18 percent, 150 g/m.sup.2 to 200 g/m.sup.2 when
the transmittance is in a range of not less than 12 percent to less
than 16 percent, 200 g/m.sup.2 to 250 g/m.sup.2 when the
transmittance is in a range of not less than 10 percent to less
than 12 percent, and 250 g/m.sup.2 to 300 g/m.sup.2 when the
transmittance is less than 10 percent.
Note that, when the recording sheet 6 is the OHP film, the light
emitted from the light emitting element 51 completely transmits
through the recording sheet 6. Meanwhile, when the recording sheet
6 has a small basis weight, that is, when the recording sheet 6 is
extremely thin, the light also transmits through the recording
sheet 6 substantially completely. Therefore, in the arrangement
where only the transmissive sensor 56 is provided, although it is
still possible to discriminate whether the recording sheet 6 is the
OHP film or a thin non-transparent sheet, the discrimination is
error-bound.
As a countermeasure, in the present embodiment, the transmissive
sensor 56 is used in conjunction with the reflective sensor 57. The
light receiving element constituting the reflective sensor 57
detects the amount of light, from the light emitting element 51,
reflected off the upper surface of the recording sheet 6, and sends
the detection signal to the CPU 47 via the received light control
circuit 55. In response to this, when the amount of light is
substantially zero, the CPU 47 decides that the recording sheet 6
is the OHP film, and when the amount of light is not substantially
zero, the CPU 47 decides that the recording sheet 6 is a
non-transparent sheet.
In this manner, by the provision of both the transmissive sensor 56
and the reflective sensor 57, it is ensured that the thickness of
the recording sheet 6 is detected, and that the OHP film and a thin
non-transparent sheet are discriminated from each other. Further,
since the reflective sensor 57 is also capable of detecting the
color of the recording sheet 6, with the described arrangement of
the sheet type detecting section 36, it is possible to discriminate
from one another the types of the recording sheet 6 having
different transparency, thickness, and color, etc.
The length of the recording sheet 6 can be detected by detecting
the transport speed of the recording sheet 6 and the time required
for the recording sheet 6 to pass through the sheet type detecting
section 36. The transport time is detected by reading a change in
transmittance and reflectance by taking the advantage of the fact
that the transmittance and reflectance are different, for example,
at the front end and rear end of the recording sheet 6.
The following will describe a control operation of the CPU 47 in
accordance with a detection signal from the sheet type detecting
section 36 referring to the flowchart of FIG. 18. Note that, the
steps are abbreviated to "S". Also, the explanation is based on the
case where the wavelength of the emitted light from the light
emitting element 51 is 840 nm.
First, when the recording sheet 6 is fed from the sheet feeding
cassette 31 or from the manual sheet feeding section 32, and the
sheet presence detecting sensor 35 detects the presence of the
recording sheet 6 (S1), the CPU 47 decides whether the amount of
light (first light quantity hereinafter) emitted from the light
emitting element 51 and the amount of light (second light quantity
hereinafter) received by the light receiving element 52 are
substantially equal to each other (S2). When it is decided that the
first light quantity and the second light quantity are
substantially equal in S2, it is decided whether the light received
by the light (third light quantity hereinafter) receiving element
53 is substantially zero (S3). When the third light quantity is
decided to be zero in S3, the CPU 47 decides that the recording
sheet 6 is the OHP film (S4).
In this manner, by examining the difference between the first and
second light quantities and the third light quantity, the CPU 47
decides whether the recording sheet 6 is the OHP film or a common
non-transparent sheet. Here, S2 may be carried out before S3;
nevertheless, the transparency of the recording sheet 6 is decided
more quickly when S2 is carried out before S3. Note that, in the
following, the recording sheet 6 of the OHP film will be referred
to as the recording sheet 6 of group 1.
Thereafter, when the first light quantity and the second light
quantity are not substantially equal in S2, or when the third light
quantity is not substantially zero in S3, the CPU 47 decides
whether the transmittance based on the difference between the first
light quantity and the second light quantity is less than 10
percent (S5). When the transmittance is decided to be less than 10
percent in S5, the CPU 47 decides that the recording sheet 6 has a
basis weight in a range of 250 g/m.sup.2 to 300 g/m.sup.2 (S6).
Note that, in the following, the recording sheet 6 having a basis
weight of less than 10 percent will also be referred to as the
recording sheet 6 of group 1, as with the OHP film.
When the transmittance is not less than 10 percent in S5, the CPU
47 decides whether the transmittance is in a range of 10 percent to
12 percent (S7). When the transmittance is in this range in S7, the
CPU 47 decides that the recording sheet 6 has a basis weight in a
range of 200 g/m.sup.2 to 250 g/m.sup.2 (S8). Note that, in the
following, the recording sheet 6 having a basis weight in this
range will be referred to as the recording sheet 6 of group 2.
When the transmittance is outside the above range in S7, the CPU 47
decides whether the transmittance is in a range of 12 percent to 16
percent (S9). When the transmittance is in this range in S9, the
CPU 47 decides that the recording sheet 6 has a basis weight in a
range of 150 g/m.sup.2 to 200 g/m.sup.2 (S10). Note that, in the
following, the recording sheet 6 having a basis weight in the above
range will be referred to as the recording sheet 6 of group 3.
When the recording sheet 6 is outside the above range in S9, the
CPU 47 decides whether the transmittance is in a range of 16
percent to 18 percent (S11). When the transmittance is in this
range in S11, the CPU 47 decides that the recording sheet 6 has a
basis weight in a range of 100 g/m.sup.2 to 150 g/m.sup.2 (S12).
Note that, in the following, the recording sheet 6 having a basis
weight in the above range will be referred to as the recording
sheet 6 of group 4. On the other hand, when the transmittance is
outside the above range in S11, the CPU 47 decides that the
recording sheet 6 has a basis weight in a range of 40 g/m.sup.2 to
100 g/m.sup.2 (S13). Note that, in the following, the recording
sheet 6 having a basis weight in the above range will be referred
to as the recording sheet 6 of group 5.
Then, the CPU 47 carries out a control of various parameters for
each of the groups 1 through 5 (S14). The various parameters
include the transfer voltage applied to the transfer roller 23 by
the power source 23a, the voltage of the power source 24a applied
to the sheet removal charger 24, a development bias voltage, a
voltage for charging the photoreceptor 7, the temperature of the
heater lamp 40a, the rotational speed of the driving motor 48 for
driving the fixing rollers, the quantity of light emitted from the
light emitting element 51 of the sheet type detecting section 36,
the image forming rate, and the transport speed of the recording
sheet 6, etc., which are controlled individually or in combination
with optimum conditions. As a result, a predetermined printing
quality in accordance with the sheet type of the recording sheet 6
is obtained.
The following will describe a control of transfer voltage by the
CPU 47, as an example of the parameter control in S 14. Note that,
the same principle also applies to the control of other
parameters.
FIG. 19 shows a relationship between the basis weight of the
recording sheet 6 and, for example, the second voltage applied to
the transfer roller 23 by the power source 23a. The CPU 47 controls
the power source 23a so that the power source 23a applies the
second voltage of, for example, as shown by the solid line b.sub.1
in FIG. 19, +600 V, +1000 V, +1300 V, +1600 V, and +2000 V to the
recording sheet 6 of group 5, group 4, group 3, group 2, and group
1, respectively to the transfer roller 23.
FIG. 20 through FIG. 22 are graphs for accessing whether the
voltage applied in accordance with the sheet type of the recording
sheet 6 is appropriate. FIG. 20 through FIG. 22 show a relationship
between second voltage and the amount of adhering toner when the
recording sheet 6 has a basis weight of 90 g/m.sup.2, 128
g/m.sup.2, and 184 g/m.sup.2, respectively. In FIG. 20 through FIG.
22, the curve c.sub.1 connecting "{character pullout}", the curve
c.sub.2 connecting ".box-solid.", and the curve c.sub.3 connecting
".tangle-solidup." represent toner layers of cyan (only one layer
of cyan), green (two layers of magenta and cyan), and black (three
layers of yellow, magenta, and cyan), respectively.
In FIG. 20 through FIG. 22, a portion of the curves steeply
ascending to the right indicates transfer failure, and a portion of
the curves steeply descending to the right indicates re-transfer
(back-transfer). Therefore, whether or not the second voltage
applied is appropriate can be decided by the portion of the curves
substantially parallel to the horizontal axis of the graphs in FIG.
20 through FIG. 22.
Specifically, in the sheet of 90 g/m.sup.2 in FIG. 20, it can be
seen that a second voltage in a range of 600 V to 1600 V, except
the portion ascending and descending to the right, is appropriate.
Here, the second voltage of +600 V applied in accordance with the
control of the CPU 47 for the recording sheet 6 of group 5 falls in
this range. Thus, it can be said that the control by the CPU 47 is
appropriate.
Similarly, in the sheet of 128 g/m.sup.2 in FIG. 21, a second
voltage in a range of 1000 V to 1500 V, except the portion
ascending and descending to the right, is appropriate. Here, the
second voltage of +1000 V applied in accordance with the control of
the CPU 47 for the recording sheet 6 of group 4 falls in this
range. Thus, it can be said that the control by the CPU 47 is
appropriate.
Also, in the sheet of 184 g/m.sup.2 in FIG. 22, a second voltage in
a range of 1000 V to 1600 V, except the portion ascending and
descending to the right, is appropriate. Here, the second voltage
of +1300 V applied in accordance with the control of the CPU 47 for
the recording sheet 6 of group 3 falls in this range. Thus, it can
be said that the control by the CPU 47 is appropriate.
Therefore, the second voltage applied by the control of the CPU 47
is in accordance with the sheet type of the recording sheet 6, and
is overall appropriate. Further, in the described voltage control,
a desirable transfer is obtained regardless of the number of toner
layers.
Note that, the values of second voltage given above are just one
example and are not limited to those. This is also true for the
first, third, and fourth voltages. Also, the second voltage may be
controlled in steps other than the described 5 steps.
In this manner, in accordance with a detection signal from the
sheet type detecting section 36, a low transfer voltage is set when
the recording sheet 6 is thin, and a high transfer voltage is set
when the recording sheet 6 is thick or is an OHP film. Thus, the
problem of transfer failure and re-transfer is prevented regardless
of the type (transparency and thickness) of the recording sheet 6.
As a result, it is possible to prevent, lowering of printing
quality as caused by different types of the recording sheet 6
used.
Note that, in the present embodiment, the second voltage is divided
into a plurality of steps in accordance with the groups to which
the recording sheet 6 belongs. However, it is possible
alternatively to have an arrangement wherein, for example, the
transmittance and corresponding basis weight of the recording sheet
6 are stored in a memory (not shown), and a control is carried out
so that a second voltage corresponding to each basis weight is
applied to the transfer roller 23.
In this case, as shown by the solid line b.sub.2 in FIG. 19, the
second voltage can be adjusted without steps, corresponding to each
basis weight. Thus, even though satisfactory transfer can be
obtained by the described step-control, with the non-step control,
it is ensured that a desirable image in accordance with the sheet
type of the recording sheet 6 is obtained, thus preventing lowering
of the printing quality with certainty.
The fixing temperature control operates in the same manner as the
transfer voltage control. That is, in accordance with a detection
signal from the sheet type detecting section 36, a low fixing
temperature is set when the recording sheet 6 is thin, and a high
fixing temperature is set when the recording sheet 6 is thick or is
an OHP film. As a result, an optimum fixing temperature is obtained
in accordance with the type of the recording sheet 6, thus
preventing the high temperature/low temperature offset when fixing
a color toner image, regardless of the type of the recording sheet
6 used.
The transport speed control of the recording sheet 6 and the image
forming rate control also operate in the same manner as the
transfer voltage control. That is, in accordance with a detection
signal from the sheet type detecting section 36, a slow transport
speed and a slow image forming rate are set when the recording
sheet 6 is thin, and a high transport speed and a high image
forming rate are set when the recording sheet 6 is thick or is an
OHP film, thus obtaining the described effect of the present
embodiment.
The following describes the fixing temperature control by the
fixing section 5 in detail.
The fixing belt 43 of the fixing section 5 has already been
maintained at a set temperature by the ON/OFF control of the heater
lamp 40a when the recording sheet 6 is transported to the fixing
section 5. The ON/OFF control of the heater lamp 40a is carried out
in accordance with the result of comparison between the length of
the recording sheet 6 as detected by the sheet type detecting
section 36 and the periphery of the fixing belt 43.
To describe in more detail, as shown in FIG. 23, when the time the
recording sheet 6 is brought into contact with the fixing belt 43
is t1, and when the time the fixing belt 43 completes one rotation
from the time the recording sheet 6 is brought into contact with
the fixing belt 43 is t 3, the CPU 47 controls the heater lamp 40a,
when deciding that the length of the recording sheet 6 is longer
than the periphery of the fixing belt 43, in such a manner that the
heater lamp 40a is turned on at any instant between t1 and t3. Note
that, when the time the heater lamp 40 a is turned on is t2, the
period of t2 to t3 is longer than the time required for conduction
of heat from the heater lamp 40a to the fixing belt 43.
On the other hand, when the CPU 47 decides that the length of the
recording sheet 6 is shorter than the periphery of the fixing belt
43, a normal fixing temperature control is carried out. The normal
fixing temperature control is the control wherein, during the
period of t1 to t3, the surface temperature of the fixing belt 43
is maintained at the set temperature by the ON/OFF control of the
heater lamp 40a, and after t3, while the heater lamp 40a is turned
on when the surface temperature drops below the set temperature so
as to avoid malfunctioning due to low temperature, the heater lamp
40a is turned off when the surface temperature exceeds the set
temperature so as to avoid malfunctioning due to high
temperature.
Note that, in FIG. 23, the time scale is different when overshoot
is observed and when it is not observed. The overshoot is a
phenomenon in which the surface temperature of the fixing belt 43
fluctuates from the set temperature by the ON/OFF control of the
heater lamp 40a.
The following describes an operation of such fixing control by the
CPU 47 referring to the flowchart of FIG. 24.
First, when feeding of the recording sheet 6 is started (S21), the
sheet type detecting section 36 detects the length of the recording
sheet 6 (S21). In the case of automatic feeding from the sheet
feeding cassette 31, the CPU 47 detects the type of the sheet
feeding cassette 31 at the time when the feeding has started and
also detects the length of the recording sheet 6, and thereafter
the sequence goes to S23.
Then, the CPU 47 decides whether the length of the recording sheet
6 is longer than the periphery of the fixing belt 43 (S23). When it
is decided that the length of the recording sheet 6 is shorter than
the periphery of the fixing belt 43 in S23, the CPU 47 decides
whether the recording sheet 6 has been inserted between the fixing
rollers 39a and 39b (S24), and carries out the normal fixing
temperature control in the described manner (S25).
Note that, the decision as to whether the recording sheet 6 has
been inserted between the fixing rollers 39a and 39b can be made by
deciding whether, for example, a particular transport time of the
recording sheet 6 from the aligning rollers 37 to the fixing
rollers 39a and 39b has been elapsed, which is stored beforehand in
a memory (not shown). Alternatively, the decision may be made by
providing a sensor, which detects the insertion of the recording
sheet 6, in the vicinity of the fixing rollers 39a and 39b.
Thereafter, in S25, under the normal fixing temperature control, a
color toner image is fixed on the recording sheet 6, and when a
sheet sensor 42 detects that the recording sheet 6 has been
discharged from the fixing rollers 39a and 39b (S26), the CPU 47
again carries out the normal fixing temperature control (S27).
Note that, in S26, discharge of the recording sheet 6 from the
fixing rollers 39a and 39b may be detected based on the length of
the recording sheet 6 as detected by the sheet type detecting
section 36, the transport distance between the aligning rollers 37
and the fixing rollers 39a and 39b, and the rotational speed of the
fixing rollers 39a and 39b. In this case, the sheet sensor 42 is
not required.
On the other hand, when it is decided by the CPU 47 in S23 that the
length of the recording sheet 6 is longer than the periphery of the
fixing belt 43, after confirming in the described manner that the
recording sheet 6 has been inserted between the fixing rollers 39a
and 39b (S28), the CPU 47 starts measuring time from the time of
insertion (time t1) (S29). At time t2 after the first predetermined
time has elapsed (S30), the CPU 47 carries out a control of turning
on the heater lamp 40a (S31). Therefore, the first predetermined
time is the time between t1 and t2, and the time t2 is the time
which is set in such a manner that the remaining period of t2 to t3
becomes longer than the time required for transfer of heat from the
heater lamp 40a to the fixing belt 43. This ensures that the heat
of the heater lamp 40a reaches the fixing belt 43 by t3, at the
latest, at which the fixing belt 43 completes one rotation, thus
suppressing the temperature drop of the fixing belt 43.
Then, after a second predetermined time has elapsed from the time
of insertion (S32), the CPU 47 carries out a control of turning off
the heater lamp 40a (S33). Note that, the second predetermined time
is the period from time t1 as a reference point to any point
between time t3 and the time the rear end of the recording sheet 6
is discharged from the fixing rollers 39a and 39b.
Thereafter, when the sheet sensor 42 detects that the recording
sheet 6 has been discharged from the fixing rollers 39a and 39b
(S34), the CPU 47 carries out again the normal fixing temperature
control (S27)
When the fixing temperature control is carried out in the described
manner, the surface temperature of the fixing belt 43 should
theoretically take the form of the curve d1 as shown in FIG. 23. In
this case, even when the length of the recording sheet 6 is longer
than the periphery of the fixing belt 43, unlike the conventional
case as indicated by the curve d2, the surface temperature of the
fixing belt 43 does not drop while the toner image is still being
fixed to the recording sheet 6. Therefore, the recording sheet 6 is
uniformly fixed from the front end to rear end, and uniform
glossiness and transparency are obtained. This effect is especially
prominent in color images. Also, in the present embodiment, the
surface temperature of the fixing belt 43 is nearly the set
temperature immediately after the recording sheet 6 is discharged,
allowing an easy start of the next fixing operation for the
recording sheet 6.
In reality, however, as shown in FIG. 25, the surface temperature
of the fixing belt 43 drops slightly after one rotation of the
fixing belt 43. Nevertheless, compared with the conventional case
as shown in FIG. 30, a decrease in the surface temperature while
fixing the recording sheet 6 is significantly reduced, thus
realizing substantially uniform fixing ability from the front end
to the rear end of the recording sheet 6.
In the present embodiment, the recording sheet 6 is fixed using the
fixing belt 43 suspended by the fixing roller 39a and the heat
roller 40. However, the same effect as obtained in the present
embodiment can also be obtained in the arrangement, as shown in
FIG. 26, wherein fixing is carried out by sandwitching the
transported recording sheet 6 by the heat roller 40 and the fixing
roller 39b without using the fixing belt 43. In this case, the heat
roller 40 constitutes the fixing means, and the periphery of the
fixing belt 43 as described above corresponds to the periphery of
the heat roller 40.
After the length of the recording sheet 6 is detected by the sheet
type detecting section 36, the CPU 47 may carry out a control of
changing the speed of each transfer step, the transport speed of
the recording sheet 6, and the set temperature of fixing, etc.,
based on the length and thickness, etc., of the recording sheet 6.
For example, when the length of the recording sheet 6 is long, the
CPU 47 carries out a control of slowing down the transport speed of
the recording sheet 6. This takes care of the case where the
conduction speed of the heat to the fixing belt 43 is slow, and as
a result uniform fixing ability is maintained from the front end to
the rear end of the recording sheet 6.
After fixing of the recording sheet 6 is finished, the CPU 47 can
carry out a control of automatically bringing the surface
temperature of the fixing belt 43 back to the set temperature
before the recording sheet 6 has passed through. This allows the
surface temperature of the fixing belt 43 to return to the set
temperature more quickly, allowing the next fixing operation for
the recording sheet 6 to be started immediately. Also, in this
case, the overshoot as described above is prevented, thus
preventing the surface temperature of the fixing belt 43 from
increasing excessively. As a result, deterioration of the fixing
belt 43 due to high temperature is prevented, thus protecting the
fixing belt 43. Here, the CPU 47 constitutes a fixing belt safe
circuit.
The CPU 47 constituting such a fixing belt safe circuit turns off
the heater lamp 40a when it is detected by the fixing temperature
sensor 41, when the recording sheet 6 is passing through the fixing
belt 43, when carrying out plural successive printing, or after the
recording sheet 6 has passed through the fixing belt 43, that the
fixing temperature is exceeding the temperature range which can be
controlled. Thereafter, the CPU 47 turns on the heater lamp 40a so
as to bring the surface temperature of the fixing belt 43 back to
the set temperature before the recording sheet 6 has passed
through. This protects the fixing belt 43 when carrying out plural
successive printing, and ensures proper fixing ability after
returning to the set temperature.
Note that, the same effects as obtained in the present embodiment
can be obtained even in the case where the photoreceptor 7 is
composed of a drum and the intermediate transfer medium 22 is
composed of a belt. Also, in the present embodiment, even though
the explanations were given through the case of full-color
printing, the same effects can be obtained in twin-color printing
and mono-color printing.
The image forming apparatus of the present invention may have an
arrangement wherein the transfer device is brought into contact
with the intermediate transfer medium in (i) the first stage before
visualization of a first color is started, (ii) the second stage
for carrying out the second transfer, and (iii) the third stage for
carrying out the third transfer for transferring a developer
adhering on the transfer medium to the intermediate transfer medium
and the fourth transfer for transferring a developer adhering on
the intermediate transfer medium to the latent image holding
device, and the transfer device applies a voltage in accordance
with each of the first, second, and third stages.
With this arrangement, the transfer device is brought into contact
with the intermediate transfer medium only in the above stages
which are required for printing, instead of contacting with the
intermediate transfer medium all the time. Therefore, it is
possible to prevent filming from generating on the surface of the
intermediate transfer medium, and the visualized image from being
transferred to the transfer device by pressure. As a result,
disturbance on the visualized image transferred to the recording
medium is prevented and it is ensured that the printing quality is
improved. Note that, filming is a phenomenon in which the developer
sticks to the intermediate transfer medium by being stretched over
at the contact between the transfer device and the intermediate
transfer medium as a result of continuous contact between these two
members.
Further, with the described arrangement, the transfer device
applies a voltage to the intermediate transfer medium in accordance
with each of the first, second, and third stages, thus ensuring
that transfer efficiency is improved in each of these stages.
The image forming apparatus of the present invention may have an
arrangement wherein the first voltage applied in the first stage
and the second voltage applied in the second stage both have the
opposite polarity to that of the developer, and the second voltage
has a larger absolute value than that of the first voltage.
With this arrangement, it is ensured that the visualized image on
the intermediate transfer medium is second-transferred to the
recording medium.
The image forming apparatus of the present invention may have an
arrangement wherein the third voltage, corresponding to the third
transfer, applied in the third stage is the voltage that is shifted
to the side of the polarity of the developer from the second
voltage applied in the second stage, and the fourth voltage,
corresponding to the fourth transfer, applied in the third stage is
the voltage that is shifted to the side of the polarity of the
developer from the third voltage, corresponding to the third
transfer, applied in the third stage.
With this arrangement, it is ensured that unnecessary developer
adhering on the transfer device is transferred back to the
intermediate transfer medium, and back to the latent image holding
device from the intermediate transfer medium.
The image forming apparatus of the present invention may have an
arrangement wherein the third voltage, corresponding to the third
transfer, applied in the third stage is equal to the fourth
voltage, corresponding to the fourth transfer, applied in the third
stage.
With this arrangement, it is ensured that unnecessary developer
adhering on the transfer device and on the intermediate transfer
medium is transferred back to the latent image holding device
simultaneously.
The image forming apparatus of the present invention may have an
arrangement wherein, in the first stage and the third stage, the
voltage applying device applies a predetermined voltage to the
transfer device for a duration longer than one rotation of the
intermediate transfer medium.
With this arrangement, the surface of the intermediate transfer
medium is uniformly charged, allowing, in the first stage, a
uniform first transfer of the visualized image on the latent image
holding device to the intermediate transfer medium, from the front
end to the rear end. Also, in the third stage, it is possible to
uniformly clean the surfaces of the transfer device and the
intermediate transfer medium.
The image forming apparatus of the present invention may have an
arrangement wherein, in the first stage, the transfer device is
separated from the intermediate transfer medium after a
predetermined voltage is applied by the voltage applying device and
before visualization of the first color is started.
With this arrangement, it is possible to prevent the visualized
image first-transferred to the intermediate transfer medium from
being disturbed by the transfer device brought into contact with
the intermediate transfer medium.
The image forming apparatus of the present invention may have an
arrangement wherein the transfer device is kept separated from the
intermediate transfer medium in the duration between the first
stage and the second stage.
With this arrangement, it is possible to prevent the visualized
image first-transferred to and overlapped on the intermediate
transfer medium from being disturbed by the transfer device brought
into contact with the intermediate transfer medium.
The image forming apparatus of the present invention may have an
arrangement wherein, in the second stage, the transfer device is
brought into contact with the intermediate transfer medium at a
timing when the front end of the visualized image on the
intermediate transfer medium coincides with the front end of the
recording medium transported between the transfer device and the
intermediate transfer medium.
With this arrangement, the front end of the visualized image on the
intermediate transfer medium coincides with the front end of the
recording medium, thus allowing second transfer of the visualized
image to the recording medium without disturbance.
The image forming apparatus of the present invention may have an
arrangement wherein, in the third stage, the transfer device
carries out the third transfer and the fourth transfer while being
in contact with the intermediate transfer medium.
With this arrangement, the transition from the third transfer to
the fourth transfer is made without separating the transfer medium
from the intermediate transfer medium, thus simplifying the control
operation of contact and separation of the transfer device.
The image forming apparatus of the present invention may have an
arrangement wherein the transfer device is separated from the
intermediate transfer medium after the fourth transfer in the third
stage.
With this arrangement, it is possible to prevent deformation of the
transfer device and the intermediate transfer medium, as caused by
the transfer device pressing the intermediate transfer medium. As a
result, it is ensured that the visualized image first-transferred
to the intermediate transfer medium is prevented from being
disturbed.
The image forming apparatus of the present invention may have an
arrangement wherein the intermediate transfer medium is composed of
an integral unit of a metal drum and a semiconductive resin
film.
With this arrangement, since the intermediate transfer medium is a
drum unit, compared with the case where the intermediate transfer
medium is composed of a belt and a plurality of suspension rollers,
the number of components can be reduced, and it is possible to
reduce the overall costs.
Further, by the integral unit of the metal drum and the
semiconductive resin film, it is ensured that the voltage applied
via the transfer device is maintained by the semiconductive resin
film, and that discharge is carried out when it is required, thus
ensuring efficient transfers.
In order to achieve the above-mentioned objects, the image forming
apparatus of the present invention includes the latent image
holding device for holding color-separated image information as an
electrostatic latent image; the plurality of developing device for
making the electrostatic latent image held by the latent image
holding device visible color by color; the intermediate transfer
medium on which a visualized image of each color visualized on a
surface of the latent image holding device is overlapped upon
contact with the latent image holding device; the transfer device,
which is separable and contactable with respect to the intermediate
transfer medium; and the voltage applying device for applying a
predetermined voltage to the transfer device, and the image forming
apparatus of the present invention may further include the cleaning
device for removing altogether a developer transferred from the
transfer device to the intermediate transfer medium and a developer
transferred from the intermediate transfer medium to the latent
image holding device, by the contact between the transfer device
and the intermediate transfer medium, and by application of the
predetermined voltage from the voltage applying device to the
transfer device, so as to clean the surface of the latent image
holding device.
With this arrangement, the electrostatic latent image formed on the
latent image holding device is made visible by the developer of
corresponding color. A plurality of visualized images obtained by
the plurality of developing devices are overlapped with one another
on the intermediate transfer medium, and thereafter are transferred
onto the recording medium from the intermediate transfer medium by
the application of a voltage from the voltage applying device to
the transfer device.
Here, a developer transferred to the intermediate transfer medium
from the transfer device and a developer transferred to the latent
image holding device from the intermediate transfer medium are
removed altogether by a single cleaning device, and therefore it is
not required to provide separate device for cleaning the transfer
means and the intermediate transfer medium.
As a result, the number of cleaning device can be reduced, thus
making the device compact and reducing the cost of the device.
The image forming apparatus of the present invention includes the
latent image holding device for holding color-separated image
information as an electrostatic latent image; the plurality of
developing device for making the electrostatic latent image held by
the latent image holding device visible color by color; the
intermediate transfer medium on which a visualized image of each
color visualized on a surface of the latent image holding device is
overlapped upon contact with the latent image holding device; the
transfer device for transferring the overlapped visualized image
from the intermediate transfer medium to the recording medium; and
the voltage applying device for applying a predetermined voltage to
the transfer device, and the image forming apparatus of the present
invention may further include the sheet type detecting device for
detecting the type of the recording medium; and the control device
for controlling an image forming operation in accordance with a
detection signal from the sheet type detecting device.
With this arrangement, the electrostatic latent image formed on the
latent image holding device is made visible by the developer of
corresponding color. A plurality of visualized images obtained by
the plurality of developing device are overlapped with one another
on the intermediate transfer medium, and thereafter are transferred
onto the recording medium from the intermediate transfer medium by
the application of voltage from the voltage applying device to the
transfer device.
Here, the control device controls the image forming operation in
accordance with a detection signal from the sheet type detecting
section. For example, the control device carries out a control of
increasing, for example, a transfer voltage applied to the transfer
device when the recording medium is thick, and carries out a
control of reducing the transfer voltage when the recording medium
is thin. As a result, it is possible to obtain desirable transfer
characteristics regardless of the thickness of the recording medium
used.
Therefore, when the control device controls, in addition to the
transfer voltage control, various parameters of image formation,
such as transport speed of the recording medium, the fixing
temperature, and the development bias voltage, in accordance with
the type of the recording medium, it is possible to carry out a
desirable image forming operation in accordance with the recording
medium.
The image forming apparatus of the present invention includes the
latent image holding device for holding color-separated image
information as an electrostatic latent image; the plurality of
developing devices for making the electrostatic latent image held
by the latent image holding device visible color by color; the
intermediate transfer medium on which a visualized image of each
color visualized on a surface of the latent image holding device is
overlapped upon contact with the latent image holding device; the
transfer device for transferring the overlapped visualized image
from the intermediate transfer medium to the recording medium; the
fixing device, which is rotatable, for fixing an unfixed visualized
image transferred on the recording medium; and heating device for
heating the fixing device so that a surface temperature of the
fixing device becomes a predetermined temperature, and the image
forming apparatus of the present invention may further include the
sheet type detecting section for detecting the type of the
recording medium; and the fixing temperature control device for
controlling the on-and-off state of the heating device in
accordance with the type of the recording medium, wherein the
fixing temperature control device carries out a control, when the
length of the recording medium detected by the sheet type detecting
device is longer than the periphery of the fixing device, so that
the heating device is turned on in advance at any instant between
the time when the recording medium is brought into contact with the
fixing device and the time when the fixing device completes one
rotation, taking into consideration the time required for
conduction of heat from the heating device to the fixing
device.
With this arrangement, the electrostatic latent image formed on the
latent image holding device is made visible by the developer of
corresponding color. A plurality of visualized images obtained by
the plurality of developing devices are overlapped with one another
on the intermediate transfer medium, and thereafter are transferred
onto the recording medium from the intermediate transfer medium.
The unfixed visualized image on the recording medium is fixed on
the recording medium by the rotation of the fixing device heated by
the heating device. Note that, the sheet type detecting devices
detects, for example, the length of the recording medium.
Incidentally, the heat of the fixing device is given off to the
recording medium or to the unfixed developer in one rotation of the
fixing device. For this reason, in the case where the recording
medium is longer than the periphery of the fixing device, while the
fixing ability is stable on the recording medium from the front end
to the point in length corresponding to the periphery of the fixing
device, the fixing ability abruptly decreases from the point past
the periphery of the fixing device to the rear end of the recording
medium.
However, in the described arrangement, the heating device is turned
on in advance at any instant between the time when the recording
medium is brought into contact with the fixing device and the time
when the fixing device completes one rotation, taking into
consideration the time required for conduction of heat from the
heating device to the fixing device. With this arrangement, the
surface temperature of the fixing device does not drop while the
unfixed image is still being fixed to the recording medium, thus
preventing lowering of fixing ability.
Therefore, the recording medium is uniformly fixed from the front
end to rear end, and uniform glossiness and transparency are
obtained. This effect is especially prominent in color images.
The invention being thus described, it will be obvious that the
same may be varied in many ways. Such variations are not to be
regarded as a departure from the spirit and scope of the invention,
and all such modifications as would be obvious to one skilled in
the art are intended to be included within the scope of the
following claims.
* * * * *